Implantable electronic medical devices and systems have been in use for a considerable period of time. One of the earliest implantable medical devices to be implanted in a recipient was a cardiac pacemaker. Other implanted electronic devices include neurostimulators, implantable sensors, defibrillators and cochlear implants. Common to all types of implantable electronic devices is the requirement that at least one electrical lead is connected thereto in order for the device or system to perform its intended function. Such a lead(s) typically terminate in one or more electrodes designed to be in contact with body tissue to deliver stimulation thereto or to sense the condition thereof.
Cochlear implants are an effective way to restore the sensation of hearing to individuals who suffer from a severe or profound hearing loss. In such cases cochlear implants bypass the hair cells of the cochlea which may be damaged or absent, and directly deliver electrical stimulation to the auditory nerve fibres of the cochlea which is representative of external sound. This electrical stimulation is then sent to the brain where it is perceived as sound.
The electrical stimulation is usually delivered by a plurality of electrodes which are inserted into the cochlea and which are strategically positioned therein, to target specific regions of the cochlea to deliver the electrical stimulation. Each of these electrodes is connected to a central implantable stimulator via a wire or wires. The implantable stimulator receives signals from an external speech processor that provide direction regarding which electrode needs to be stimulated and at what frequency and amplitude. The implantable stimulator then sends the appropriate stimulation to the appropriate electrode to reproduce the desired sound sensation.
A considerable amount of research has been undertaken in the area of understanding the way sound is naturally processed by the human auditory system. With such an increased understanding of how the cochlea naturally processes sounds of varying frequency and magnitude, it has been possible to identify areas of improvement in delivering electrical stimulation to the auditory nerve to take into account the natural characteristics of the cochlea.
It is known in the art that the cochlea is tonotopically mapped. In other words, the cochlea can be partitioned into regions, with each region being responsive to signals in a particular frequency range. This property of the cochlea is exploited by providing the electrode assembly with an array of electrodes, each electrode being arranged and constructed to deliver a cochlea stimulating signal within a preselected frequency range to the appropriate cochlea region. The electrical currents and electric fields from each electrode stimulate the cilia disposed on the modiola of the cochlea. Several electrodes may be active simultaneously.
It has been found that in order for these electrodes to be effective, the magnitude of the currents flowing from these electrodes and the intensity of the corresponding electric fields, are a function of the distance between the electrodes and the modiola. If this distance is relatively great, the threshold current magnitude must be larger than if the distance is relatively small. Moreover, the current from each electrode may flow in all directions, and the electrical fields corresponding to adjacent electrodes may overlap, thereby causing cross-electrode interference. In order to reduce the threshold stimulation amplitude and to eliminate cross-electrode interference, it is advisable to keep the distance between the electrode array and the modiola as small as possible. This is best accomplished by providing the electrode array in the shape which generally follows the shape of the modiola. Also, this way the delivery of the electrical stimulation to the auditory nerve is most effective as the electrode contacts are as close to the auditory nerves that are particularly responsive to selected pitches of sound waves.
In order to achieve this electrode array position close to the inside wall of the cochlea, the electrode needs to be designed in such a way that it assumes this position upon or immediately following insertion into the cochlea. This is a challenge as the array needs to be shaped such that it assumes a curved shape to conform with the shape of the modiola and must also be shaped such that the insertion process causes minimal trauma to the sensitive structures of the cochlea. In this sense it has been found to be desirable for the electrode array be generally straight during the insertion procedure.
Several procedures have been adopted to provide an electrode assembly that is relatively straightforward to insert while adopting a curved configuration following insertion in the cochlea. In one case, a platinum wire stylet is used to hold a pre-curved electrode array in a generally straight configuration up until insertion. Following insertion, the platinum stylet is withdrawn allowing the array to return to its pre-curved configuration.
Other methods have also been proposed and implemented with varying degrees of success. Such methods include constructing the array in a straight configuration and inserting positioners with the electrode array to force the array into its final position close to the modiola. Such positioners may be designed to fill up the space within the cochlea and behind the array, so that the array is forced against the inner wall of the cochlea. Examples of such designs can be seen in U.S. Pat. Nos. 6,195,586 and 6,163,729. It is considered that such methods are not ideal as they are invasive and tend to damage the sensitive structures of the cochlea, and should there be a need to remove the electrode array from the cochlea, it would be difficult to do so with such devices without causing severe damage to the cochlea.
Other methods of achieving close positioning of the array to the inner wall of the cochlea have been proposed which utilise bioresorbable polymers and the like which swell upon contact with fluid to force the array into position. Also there are other methods which utilise a dissolvable layer which dissolves upon contact with fluid to release positioning fins to assist in positioning the array. Such methods are shown in U.S. Pat. No. 5,653,742. One problem with such methods is that it is difficult to control the dissipation/swelling of the bioresorbable polymers and as such full and total control of the shape of the array during insertion is difficult to achieve.
The present invention aims to improve on the above mentioned prior art by providing a cochlear implant array that is capable of being shaped controlled during the insertion process so that the array can achieve close positioning to the modiolus as well as achieving minimal damage to the sensitive structures of the cochlea.
In other types of implantable medical devices, particularly cardiac pacemakers and neurostimulators, the electrodes must be strategically positioned in the body close to desired stimulation sites, namely heart tissue or nerve sites. In this regard it is important that if the electrode requires passing through tissue or regions of the body such as arteries or veins, the shape of the electrode element be controlled to ensure that this occurs without considerable damage to the surrounding tissue. In another aspect therefore, the present invention aims to provide an implantable conductor that is capable of being shape controlled or steered during the insertion process so that the array can be strategically position close to the desired stimulation site without causing surrounding damage to the surrounding tissue.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed in Australia before the priority date of each claim of this application.